Carbon nanotube based field effect transistors (CNTFETs) show great promise for device applications. Recently CNTFETs with excellent electrical characteristics comparable to state-of-the-art silicon MOSFETs have been demonstrated [see, for example, Rosenblatt et al., “High Performance Electrolyte-Gates Carbon Nanotube Transistors”, NanoLetters, 2(8), (2002) pp. 869-72]. The electrical characteristics of CNTFETs however depends largely on the band-gap of the single wall carbon nanotube (SWNT) forming the channel of the transistor. Since the band-gap of SWNTs has a strong dependence on the diameter, accurate control of the diameter is essential to the success of any device technology based on carbon nanotubes.
A widely used technique for the growth of SWNTs with a narrow diameter distribution is based on the laser ablation of a graphite target. However controlled placement and orientation of tubes produced by laser ablation has proved extremely challenging, and limits the usefulness of this technique for circuit applications. A more attractive approach for circuit integration is to grow the carbon nanotubes in place on a suitable substrate using chemical vapor deposition (CVD). Various studies have shown the feasibility of controlling the orientation and origin of CVD grown carbon nanotubes using substrates with patterned catalyst [see, for example, Huang et al., “Growth of Millimeter-Long and Horizontally Aligned Single-Walled Carbon Nanotubes on Flat Substrates”, J. American Chemical Soc., 125(2003)pp.5636-5637].
A crucial difficulty in obtaining individual SWNTs by CVD is control of nanometric catalyst particle size at growth temperatures of 700-1000° C. It has been theorized that the particle size of the growth catalyst used can define the diameter of as grown carbon nanotubes. This hypothesis has been supported by the observation that catalytic particles at the ends of CVD grown SWNT have sizes commensurate with the nanotube diameters [see, for example, Li et al., “Growth of Single-Walled Carbon Nanotubes from Discrete Catalytic Nanoparticles of Various Sizes”, J. Physical Chemistry, 105 (2001), pp. 11424-11431]. Catalysts typically employed are transition metals, notably Fe, Mo, Co, NI, Ti, Cr, Ru, W, Mn, Re, Rh, Pd, V or alloys thereof. However, the synthesis of small catalyst particles with a narrow diameter distribution is complicated and difficult to control.
Numerous strategies have been employed to control catalyst size and thus CNT diameters. One of the difficulties is to prevent catalyst agglomeration during growth of the CNT at elevated temperatures. One strategy has been to embed the catalyst particles in a high surface area silica or aluminosilicate matrix, which does not sinter during heating as detailed by Cassell et al., J. Am. Chem. Soc. 1999, 121, 7975-7976. Although a high yield of SWNT is obtained with the catalyst matrix, the matrix is extremely rough and nonconductive making patterning and electrical contact difficult. Other approaches to control the catalyst particle size include the preparation of dilute discrete nano particles in solution utilizing ferrite (iron storage protein) micelles, polymers or other coordinating reagents that bind the surface of the nanoparticles and prevent them from growing bigger, as described by Li et al., supra. However, the synthesis of dilute discrete nano particles in solution suffers from the difficulty of tightly controlling particle size and limiting their agglomeration during CNT growth. Additionally, the dilute nature of the mixtures typically results in a lower SWNT yield, presumably due to the lower density of active sites. Other difficulties include the need to remove excess organic material surrounding the metal nanoparticle by pyrolysis and to reduce the oxide to activate the catalyst.
Another drawback of controlling the carbon nanotube diameter by catalyst particle size is the inability to define sufficiently small catalyst particles by lithography. Sub 1 nanometer dimensions are beyond the realm of even e-beam lithography and preclude the possibility of lithography patterning individual catalyst particles for growth of CNTs with small diameter. Thus growth of thin nanotubes (<5 nanometers) from large catalyst particles (>20 nanometers) which can be patterned lithographically has numerous advantages from an integration standpoint.
One of the main challenges facing carbon nanotube based electronics is the low drive currents of present-day CNTFETs. The low drive current stems from the extremely small diameter of SWNTs effectively resulting in a transitor with a narrow width. Using arrays of SWNTs for the channel region will increase the drive current, making CNTFET based technologies feasible. However, at the present time no controlled ways exist forming arrays of CNTs with a well defined pitch. Thus the ability to grow arrays of SWNTs with lithographically controlled origins (limited by ebeam resolution) and small diameters (<5 nanometers) is crucial to the success of CNT electronics.